Early detection of breast cancer and other types of cancer is typically an important factor in successful treatment. Ultrasound tomography is a promising imaging modality that has the potential to improve medical imaging of tissue for screening and diagnosis purposes compared to conventional imaging techniques. For instance, mammography is the current standard for breast screening, but involves ionizing radiation that precludes frequent imaging, and mammography has low sensitivity for detection of cancer in patients with dense breast tissue, which leads to a relatively high false negative rate. As another example, magnetic resonance imaging (MRI) is prohibitively expensive for routine and also has limited accessibility.
The basic principle of ultrasound involves emitting an acoustic wave or beam along a focused path from a source transmitter, and allowing the wave to scatter (e.g. in reflection, refraction, diffraction, transmission) from tissue or other boundaries in its path. The scattered wave returns to a surface of receiving elements, which is usually centered around and including the transmitter. The time of translation is converted into a depth distance by, multiplying the time by an assumed constant speed of sound in the media. The received signal is output to a graphical display for user interpretation.
However, current methods of ultrasonic imaging have some drawbacks and limitations. The assumption of a constant sound speed of propagation can cause angular displacement of objects due to refractions of the beam at boundary interfaces, and result in boundaries imaged closer than or further away from their actual positioning. Furthermore, to compensate for signal decay due to attenuation in tissue (energy loss due to scatter and energy absorption), conventional ultrasound systems incorporate automatic gain compensation (AGC) and/or time gain compensation (TGC) which allow a user to adjust signal compensation with respect to depth distance and provide a hardcoded gain to compensate for presupposed minimum signal decay. However, these compensations lead to another form of artifact caused by media of varying attenuation, which can degrade the image quality. If an object of attenuation lower than that assumed by the compensation techniques lies in the path of the beam, the resulting image includes a brightening of hyperechoic tissue behind the object. Similarly, if an object of attenuation higher than that assumed by the compensation techniques lies in the path of the beam, the resulting image includes a shadow of hypoechoic tissue behind the object.
Thus, there is a need in the ultrasound imaging field to create an improved method for imaging a volume of tissue. This invention provides such an improved method for imaging a volume of tissue.